ClickHouse/dbms/include/DB/Storages/MergeTree/MergeTreeReader.h

#pragma once

#include <DB/Storages/MarkCache.h>
#include <DB/Storages/MergeTree/MarkRange.h>
#include <DB/Storages/MergeTree/MergeTreeData.h>
#include <DB/Core/NamesAndTypes.h>


namespace DB
{

class IDataType;
class CachedCompressedReadBuffer;
class CompressedReadBufferFromFile;


/// Reads the data between pairs of marks in the same part. When reading consecutive ranges, avoids unnecessary seeks.
/// When ranges are almost consecutive, seeks are fast because they are performed inside the buffer.
/// Avoids loading the marks file if it is not needed (e.g. when reading the whole part).
class MergeTreeReader : private boost::noncopyable
{
public:
	using ValueSizeMap = std::map<std::string, double>;

	MergeTreeReader(const String & path, /// Path to the directory containing the part
		const MergeTreeData::DataPartPtr & data_part, const NamesAndTypesList & columns,
		UncompressedCache * uncompressed_cache,
		MarkCache * mark_cache,
		bool save_marks_in_cache,
		MergeTreeData & storage, const MarkRanges & all_mark_ranges,
		size_t aio_threshold, size_t max_read_buffer_size,
		const ValueSizeMap & avg_value_size_hints = ValueSizeMap{},
		const ReadBufferFromFileBase::ProfileCallback & profile_callback = ReadBufferFromFileBase::ProfileCallback{},
		clockid_t clock_type = CLOCK_MONOTONIC_COARSE);

	~MergeTreeReader();

	const ValueSizeMap & getAvgValueSizeHints() const;

	/// If columns are not present in the block, adds them. If they are present - appends the values that have been read.
	/// Do not adds columns, if the files are not present for them (to add them, call fillMissingColumns).
	/// Block should contain either no columns from the columns field, or all columns for which files are present.
	void readRange(size_t from_mark, size_t to_mark, Block & res);

	/// Add columns from ordered_names that are not present in the block.
	/// Missing columns are added in the order specified by ordered_names.
	/// If at least one column was added, reorders all columns in the block according to ordered_names.
	void fillMissingColumns(Block & res, const Names & ordered_names, const bool always_reorder = false);

	/// The same as fillMissingColumns(), but always reorders columns according to ordered_names
	/// (even if no columns were added).
	void fillMissingColumnsAndReorder(Block & res, const Names & ordered_names);

private:
	class Stream
	{
	public:
		Stream(
			const String & path_prefix_, const String & extension_, size_t marks_count_,
			const MarkRanges & all_mark_ranges,
			MarkCache * mark_cache, bool save_marks_in_cache,
			UncompressedCache * uncompressed_cache,
			size_t aio_threshold, size_t max_read_buffer_size,
			const ReadBufferFromFileBase::ProfileCallback & profile_callback, clockid_t clock_type);

		static std::unique_ptr<Stream> createEmptyPtr();

		void seekToMark(size_t index);

		bool isEmpty() const { return is_empty; }

		ReadBuffer * data_buffer;

	private:
		Stream() = default;

		/// NOTE: lazily loads marks from the marks cache.
		const MarkInCompressedFile & getMark(size_t index);

		void loadMarks();

		std::string path_prefix;
		std::string extension;

		size_t marks_count;

		MarkCache * mark_cache;
		bool save_marks_in_cache;
		MarkCache::MappedPtr marks;

		std::unique_ptr<CachedCompressedReadBuffer> cached_buffer;
		std::unique_ptr<CompressedReadBufferFromFile> non_cached_buffer;

		bool is_empty = false;
	};

	using FileStreams = std::map<std::string, std::unique_ptr<Stream>>;

	/// avg_value_size_hints are used to reduce the number of reallocations when creating columns of variable size.
	ValueSizeMap avg_value_size_hints;
	String path;
	MergeTreeData::DataPartPtr data_part;

	FileStreams streams;
	size_t cur_mark_idx = 0; /// Mark index corresponding to the current position for all streams.

	/// Columns that are read.
	NamesAndTypesList columns;

	UncompressedCache * uncompressed_cache;
	MarkCache * mark_cache;
	/// If save_marks_in_cache is false, then, if marks are not in cache, we will load them but won't save in the cache, to avoid evicting other data.
	bool save_marks_in_cache;

	MergeTreeData & storage;
	MarkRanges all_mark_ranges;
	size_t aio_threshold;
	size_t max_read_buffer_size;

	void addStream(const String & name, const IDataType & type, const MarkRanges & all_mark_ranges,
		const ReadBufferFromFileBase::ProfileCallback & profile_callback, clockid_t clock_type,
		size_t level = 0);

	void readData(
			const String & name, const IDataType & type, IColumn & column,
			size_t from_mark, size_t max_rows_to_read,
			size_t level = 0, bool read_offsets = true);

	void fillMissingColumnsImpl(Block & res, const Names & ordered_names, bool always_reorder);
};

}